RSLogix 500... What's your take on this rung?

AutomationTechBrian · Aug 7, 2019

The thing about troubleshooting with the ladder logic... you get to see other techniques. I ran across this yesterday and I'm thinking about the purpose. The ladder was uploaded from the plc, so there are no comments.

I start explaining rung 40... my first thought is insurance on the "one shot", but then the B19:1/4 used as both contacts and coil screw me up.

Steve Bailey · Aug 7, 2019

It's classic flip-flop logic.
Each time B20:1/9 turns on B19:1/4 changes state.

AutomationTechBrian · Aug 7, 2019

Ok.... I see it now. Thanks!

Firejo · Aug 7, 2019

It's a toggle routine.
B20:1/9 goes high sending B3:0/3 high and if B19:1/4 is low it goes high. If it is high it goes low.

BruceJW · Aug 7, 2019

yes, without looking at following comments, I said FLIP FLOP

parky · Aug 7, 2019

On low to high of first contact, creates a one shot, this then sets the output on rung 2, on the next transition of low to high on rung 1 it de-latches the output so this is basically a flip flop could be used for a start/stop circuit from one push button
I.e. First push puts output on, on second push turns output off

parky · Aug 7, 2019

Same here I was actually writing it but others beat me to the post.

roxusa · Aug 7, 2019

I have used this in every program I've ever written.

AutomationTechBrian · Aug 7, 2019

That's what I love about this place. I may work alone, but I'm never really alone.

blftech · Aug 7, 2019

I see this used when a single button on an HMI is used as the ON/OFF button.

Each time you press the button it changes state. It can be handy when you have limited screen space on the HMI.

You can change the button color to show the current state.

Ken Moore · Aug 7, 2019

I have used similar logic with the done bit on a self-resetting timer. Delete rung 39 and use the done bit for B3:0/3. With a 0.5 sec timer, the bit changes state each half second, then you can use this to flash warning lights and such.

Aabeck · Aug 7, 2019

blftech said:
I see this used when a single button on an HMI is used as the ON/OFF button....

I have used this with physical, illuminated pushbuttons on panels with limited space, or only one opening left in an operator pendant.

EDIT: Generally good to add a debounce timer on physical switches triggering a PLC OneShot.

RSLogix 500... What's your take on this rung?

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